Genital Cancer in Horses Linked to Newly Discovered Virus

A newly discovered virus might be a cause of equine genital cancer, an aggressive type of skin cancer that affects male and female horses of all breeds, according to Tim Scase, BSc, BVM&S, PhD, MRCVS, Dipl. ACVP, director of Bridge Pathology Ltd., a diagnostic immunohistochemistry laboratory in the United Kingdom. Equine genital cancer is thought to be the second most common cancer in horses, Scase said.

Scase and his colleagues examined tissue samples from genital tumors in both male and female horses and discovered a new papillomavirus that they called "EcPV-2." They found this virus in most of the cancerous tissue samples that they tested, and it was not present in any of the healthy test tissues.

"These results demonstrate for the first time that a previously undiscovered virus is likely causing one of the most serious cancers that affect horses," Scase said.

Scase, a veterinary pathologist, said that he often sees tissue characteristics that resemble those of warts (Scase defines warts as "a benign tumor growing out from the skin rather than invading the underlying tissues") when he looks at equine genital cancer tissue samples through a microscope. This made him think that a papillomavirus might be behind these particular cancerous growths.

Papillomaviruses have been linked to both genital warts and genital cancers in people, Scase said: "It seemed plausible that a similar virus might be causing these warts in horses, and that the more aggressive cancers might develop as a result of the viral infection.

"In the EcPV-2 case, the progression from benign wart to malignant squamous cell carcinoma is likely through the effects of persistent viral infection and other factors, such as exposure to smegma," Scase explained. Smegma is the thick, oily, or cheesy secretion that collects beneath the sheath.

Warts in horses--benign or malignant--are a result of the equine papilloma virus. Papillomaviruses, specifically the bovine strain, have also been linked to equine sarcoids, a common equine skin tumor for which there is currently no cure.

"Potentially, a veterinarian may be able to surgicaly remove the warts before there is any progression to an invasive squamous cell carcinoma (a tumor that has invaded the underlying tissues and has significant potential for spread to other organs) or to a carcinoma in situ (a pre-invasive stage of the tumor where the cells have not yet invaded into the underlying tissues)," Scase said. "However, as the virus can potentially infect cells next to the wart,.if the whole thing is removed, there may be residual virus hiding in the cells left behind that can then become reactivated and result in further wart formation or the more invasive tumor types.

"The diseases are virtually identical in horses and people," Scase added. "The warts and the cancers look the same under the microscope, and the papillomavirus that we found is similar at a genetic level to the papillomaviruses that cause the same cancers in people.

"Horse owners and veterinarians treating a horse with genital warts should be aware that these horses may be at greater risk of developing squamous cell carcinomas at a later date," he added.

According to Scase, this ground-breaking discovery has opened the door to new preventive strategies. "Now we have found the virus, it should be possible to develop a vaccine against it to prevent these cancers developing in later life."

The study, "Equus caballus papillomavirus-2 (EcPV-2): an infectious cause for equine genital cancer?" was published in the November 2010 Equine Veterinary Journal.The abstract is available on PubMed.